This invention relates to piezoelectric transducer devices for converting electrical energy to ultrasonic energy, or vice versa.
Piezoelectric transducers are in widespread use for generating ultrasonic energy from electrical signals, and/or for converting ultrasonic energy back into electrical form. Application of a voltage across surfaces of a piezoelectric material produces mechanical distortion of the material which, in turn, launches wave energy such as ultrasound. Conversely, the application of ultrasound to such materials produces an electrical polarization.
Ultrasonic energy is utilized in systems for imaging the internal structure of the human body, inspecting equipment and parts, and many other uses. Ultrasonic energy tends to be reflected at interfaces at which the acoustic impedance of a transmission medium changes from one value to another. It is therefore desirable to have similarity of acoustic impedance of a transducer, the medium through which ultrasound is to travel to or from a body to be examined, and the body itself. This results in a more efficient communication of energy to and/or from the body, and the minimization of unnecessary losses of energy that would be expected at severe discontinuities of the acoustic impedance.
Piezoelectric plastic, for example, polyvinyladine fluoride ("PVDF") has certain desirable properties for use as a piezoelectric transducer. For example, the material is less expensive and easier to use than certain crystalline or ceramic piezoelectric materials, and it has an acoustic impedance which is near that of water and which closely matches the acoustic impedance of plastic ultrasound focusing lenses. It is therefore suitable, in this regard, for use when transmitting to or from a body with a high water content, such as in medical imaging applications.
In general, however, piezoelectric plastics have a relatively low piezoelectric coupling coefficient, which is the measure of the material's efficiency of converting electrical energy to mechanical energy, or vice versa. This can tend to be a limiting factor on their usefulness from the standpoint of sensitivity or, at least, the bandwidth over which sensitivity is sufficient for a given application.
It is among the objects of the present invention to provide a piezoelectric transducer device which exhibits improved sensitivity and bandwidth performance.